EMBRYONIC INDUCTORS AND ORGANIZERS 499 



but not until later stages when transplanted to the trunk. This difference 

 is attributed to the mesoderm of the head region (Harrison, 1925^). The 

 anterior part of the archenteric roof placed in the blastocoel of the early 

 Triton gastrula comes to lie ventrally and induces balancer there. Al- 

 though the axolotl possesses no balancer or only traces, inductor trans- 

 plants to Triton may induce balancer development (Mangold, 193 1&). 

 Heteroplastic transplants of presumptive balancer ectoderm from 

 A . punctatum to the corresponding region of A . tigrimim, which has no bal- 

 ancer, may develop balancer; but ectoderm of A. tigrinum., transplanted 

 to the balancer region of A. punctatum, does not give rise to balancer 

 (Harrison, 1925&). Heteroplastic transplants of presumptive balancer ec- 

 toderm and of ventral ectoderm to the balancer region between Triton 

 species give balancers characteristic of the donor species. The reaction, 

 but not the inducing factor, is species-specific (Rotmann, 1934, 19356). 

 Neural crest material of R. fusca in the blastocoel of Triton and axolotl 

 may induce neural plate and various modifications in the host, among 

 them often supernumerary balancers, but always in the balancer region 

 (Raven, 1931, 1933&). 



The urodele mouth has small teeth, the anuran mouth horny jaws; 

 in anuran larvae two suckers are present ventral to the mouth, but no 

 balancers. The entoderm underlying the presumptive oral ectoderm is 

 necessary for formation of the oral region in Triton and axolotl. Presump- 

 tive oral ectoderm of neurulae, transplanted or explanted alone, does not 

 form a mouth (Stroer, 1933). Presumptive abdominal ectoderm of the 

 anuran gastrula, transplanted to the urodele mouth region, gives rise to 

 an anuran mouth with horny jaws and suckers; abdominal urodele ecto- 

 derm in the anuran mouth region develops a urodele mouth with teeth, 

 and, if the transplant includes the balancer region, balancers, sometimes 

 supernumerary, develop. Both balancer and sucker may develop in some 

 cases, one from the host, the other from the donor ectoderm; and, if the 

 boundary between urodele and anuran ectoderm is in the region of the 

 anuran sucker, part of a sucker develops on the anuran side, none on the 

 urodele side, the development ceasing sharply at the junction.''^ 



Presumptive anuran gill ectoderm, transplanted alone to other regions 

 in neurula or tail-bud stages, develops gill stumps, and the gill pattern 

 in transplants turned 180° is similarly turned. Ectoderm adjoining the 

 gill region is also capable of gill development (Ekman, 1913a, b; 1922). 



■"Spemann und Schotte, 1932; Schotte, 1932, 1933; Rotmann, 19350.; Holtfreter, 1936; 

 G. A. Schmidt, 1937a. 



